Learn Croatian: Gdje su pisma? - Where are the letters?

In this case, -a is the plural ending for a neuter noun. The singular word is pismo (letter). While feminine singular words do typically end in -a, regular neuter nouns drop their -o or -e ending and add -a when there is more than one. So, pisma simply means letters.

Why do we use the verb su (are) here?

Because pisma is plural! Even though the -a ending might trick you into thinking it's a singular feminine noun, it is actually multiple letters. Since we are asking about more than one thing, we must use the plural verb su instead of the singular je.

There is no word for "the" in this sentence. How do we know it means "the letters"?

Croatian does not have definite or indefinite articles (words like "a", "an", or "the"). You simply figure it out from context. Asking "Where are letters?" generally doesn't make sense in English, so we naturally add "the" in the translation.

Does pisma mean letters you send in the mail, or letters of the alphabet?

Pisma refers to written messages you send in the mail. If you want to talk about letters of the alphabet, you would use the word slovo (which is also a neuter noun, so its plural is slova!).